This invention relates to a solder preform having high melting point metal particles uniformly dispersed therein and a process for its manufacture.
A typical soldering method for constituent parts such as printed circuit boards and electronic parts used in electronic equipment (referred to below as electronic parts or simply as parts) is the reflow soldering method.
The reflow soldering method is a method in which solder is placed only on necessary locations of a part, and then heating is performed by a heating apparatus such as a reflow furnace, an infrared irradiating apparatus, or a laser irradiating apparatus to carry out soldering. The reflow method not only has excellent productivity but can also carry out soldering with excellent reliability in that solder is not adhered to unnecessary locations. Therefore, it is much used in soldering of modern electronic parts which require high reliability.
Solders which are used in the reflow method include solder paste and solder preforms. Solder paste is formed by mixing a viscous flux and solder powder. It is applied by printing or dispensing to portions to which electronic parts are soldered. Flux which is used in solder paste has solids such as rosin and an activator dissolved in a solvent. Therefore, flux residue always adheres to soldered portions after soldering with a solder paste. If flux residue absorbs moisture in the atmosphere, it may produce corrosive products in soldered portions or a deterioration in insulation resistance. Therefore, parts which are soldered with solder paste must generally undergo cleaning of flux residue when high reliability is demanded.
In order to solder electronic parts requiring high reliability, solder preforms which enable soldering without employing flux are used. A solder preform is solder which is previously formed (preformed) into a shape such as a pellet or a washer suited to the portion to be soldered. In a reflow method using a solder preform, the solder preform is placed on a portion to be soldered of an electronic part, and then the part is heated in a reducing atmosphere such as hydrogen gas to perform soldering. If an electronic part having a solder preform placed thereon is heated in a hydrogen atmosphere, the hydrogen acts to reduce and remove oxides adhering to the surface of the portion to be soldered of the part and the surface of the solder preform, thereby allowing the molten solder to wet the surface of the portion to be soldered.
A typical soldering technique which uses a solder preform is die bonding. Die bonding is joining of electronic parts such as a substrate and a semiconductor element with solder. Soldering is carried out by placing a solder preform between the substrate and the semiconductor element followed by heating in a reducing atmosphere.
If parts demanding high reliability are soldered with a solder preform without using flux, the problem of corrosion due to absorption of moisture by flux residue does not occur. Even so, corrosion of soldered portions sometimes becomes a problem. This type of corrosion is due to moisture condensation. If the periphery of a soldered part is subjected to a heat cycle of high temperatures and low temperatures, when the temperature of the part decreases from a high temperature to a low temperature, moisture in the periphery of the part condenses, and water droplets adhere to the soldered portion. In a soldered portion, the ionization tendency of the solder alloy is different from that of the metal of the portion to be soldered of the electronic part. As a result, the adhered water droplets dissolve electrolytes and form a local cell, and the solder or the metal of the part may corrode. In order to prevent corrosion due to moisture in parts demanding high reliability, resin molding or potting in which the entire part is covered with a resin is carried out.
When a solder preform and a semiconductor element are placed on a substrate and heated so that the solder preform melts, the molten solder may be forced out from the desired soldered portions of the parts due to the weight of the semiconductor element, parts such as heat sinks, and jigs or the like, and the amount of solder present between the portions to be soldered may end up becoming small. Joining by soldering can provide a sufficient bonding strength to the extent that a suitable amount of solder is present between portions to be soldered, but if solder is forced out from between portions to be soldered by the weight of a semiconductor element as in die bonding and the amount of solder becomes small, the bonding strength becomes weak.
In order to provide a suitable clearance between portions to be soldered and ensure that a suitable amount of solder is present between portions to be soldered, a technique has been employed in which a plurality of spherical particles of a high melting point metal having a melting point higher than solder such as Ni, Cu, Ag, Fe, Mo, and W (referred to below simply as metal particles) are sandwiched between portions to be soldered. For this purpose, solder preforms which already have metal particles dispersed therein have been used, since it is extremely troublesome and inefficient to separately place discrete metal particles between portions to be soldered at the time of soldering.
Methods of manufacturing solder preforms having metal particles dispersed therein include the pressing method and the melting method.
In the pressing method, a large number of metal particles are placed atop a single solder sheet, the sheet is passed between a pair of rollers to embed the metal particles in the solder sheet, and the sheet is then subjected to punching with a press (see JP H03-281088 A1). Alternatively, metal particles are placed between two solder sheets to form a sandwich, which is then subjected to punching with a press (see JP H06-285686 A1, for example).
In the melting method, metal particles are dispersed in molten solder, and the molten solder is then cast into a mold to form a billet. The billet is extruded to form a solder sheet, and the sheet undergoes punching with a press (see JP H06-31486 A1, for example). In the melting method disclosed in JP H06-31486 A1, the surface of metal particles is first treated by electroplating or electroless plating. A mixture of the metal particles and flux is then charged into molten solder and stirred, and then the molten solder is cast into a mold to form a billet. The billet is then rolled to form a sheet, and the sheet is formed into solder preforms of a predetermined shape with a press.
Because a solder preform which is obtained by the pressing method has metal particles mechanically embedded in a solder sheet or sandwiched between solder sheets, the metal particles have not been wet by molten solder. Namely, the metal particles and the solder are not metallically bonded to each other. Therefore, if such a solder preform is placed between portions to be soldered of electronic parts and the solder preform is melted, a metallic bond is not formed where the metal particles and the portions to be soldered are merely touching. This state decreases the bonding area between the metal particles and solder and causes voids. As a result, not only is the bonding strength inadequate, but the heat dissipation capacity decreases.
Heat dissipation capacity as used herein refers to, in the case of soldering of an electronic part such as a power transistor to a heat sink, for example, the ability to efficiently release heat, which is generated by the electronic part, through the heat sink. Heat dissipation capacity is greatly affected by thermal conductivity in the soldered portions. By improving the heat dissipation capacity, a deterioration in performance of an electronic part due to a temperature increase of the part is prevented.
Particularly in the case of such a heat generating part, if there is not complete bonding between the electronic part and the heat sink, the bonding area becomes small and voids develop, thereby making heat conduction inadequate, and this produces thermal effects on electronic parts.
In solder preforms manufactured by the conventional melting method, since metal particles have been mixed with flux before they are charged into molten solder, it was expected that the metal particles and the solder are metallically bonded to each other to obtain a sufficient bonding strength. However, the bonding strength after soldering was insufficient. When a solder preform obtained by the conventional melting method is used for soldering by sandwiching it between parts and the interior of the soldered portions is observed with an x-ray transmission apparatus, voids which were not visible prior to soldering appear after soldering.
If voids develop in soldered portions, in the same manner as with solder preforms manufactured by the pressing method in which metal particles are not metallically bonded to solder, the bonding area becomes small, and not only does the bonding strength and the heat dissipation capacity decrease, but voids expand due to the heat at the time of soldering and parts sometimes end up tilting.
With solder preforms manufactured by the conventional melting method, it has been sometimes observed after soldering that flux oozes out to the periphery of soldered portions. If flux oozes out to the periphery of soldered portions, the flux causes corrosion. In addition, when resin molding or potting is carried out in order to protect soldered portions against moisture, the flux is mixed into the resin and may interfere with curing of the resin.
In light of the above circumstances, there exists a need for a solder preform containing metal particles which does not develop voids at the time of solder bonding of parts and which does not experience a decrease in strength or a decrease in corrosion resistance and a process for its manufacture.